Specific and selective degradation of protein by illumiation

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A photosystem II reaction center D1 protein was slelctively and specifically degraded during the course of strong light illuminaition, photoimhibition. The loss of the function of Q_A was primarily responsible for the loss of the photosystem II function by photoinhibition. The photoinhibition was sppressed or stimulated by the presence of varios type photosytem II inhibitors, suggesting that the changes in the Q_B-binding site by the binding of the inhibitors modulates the degradation of the D1 protein. In fact, the cleavage of the D1 protein was prrformed under complete darkness in the presence of photosystem II inhibitor, PNO8 (N-octyl-3-nitro-2,4,6-trihydroxy benzamide). The cleavage was inhibited in the presence of other PS II inhibitor, DCMU,indicating that the binding of PNO8 in the Q_B-site is responsible for the cleavage reaction. The cleavage was performed at one site in the vicinity of Lue 258 in the de-loop region of the D1 protein to give 23 and 9 kDa fragments that are also found during photoinhibition by illuminating strong light. The cleavege was inhibiet under low temperature and inhibitors of serine type protease but was not affected by the presence of oxygen. The cleavage did not require light but was triggered by short time pre-illumination. The results suggest that some changes in the Q_B-binding site by illumination and the binding of PNO8 induce the state which allows the cleavage reaction of the D1 protein.